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.TH SIGALTSTACK 2 "Nov 1, 2003"
.SH NAME
sigaltstack \- set or get signal alternate stack context
.SH SYNOPSIS
.LP
.nf
#include <signal.h>

\fBint\fR \fBsigaltstack\fR(\fBconst stack_t *restrict\fR \fIss\fR, \fBstack_t *restrict\fR \fIoss\fR);
.fi

.SH DESCRIPTION
.sp
.LP
The \fBsigaltstack()\fR function allows a thread to define and examine the
state of an alternate stack area on which signals are processed. If \fIss\fR is
non-zero, it specifies a pointer to and the size of a stack area on which to
deliver signals, and informs the system whether the thread is currently
executing on that stack.  When a signal's action indicates its handler should
execute on the alternate signal stack (specified with a \fBsigaction\fR(2)
call), the system checks whether the thread chosen to execute the signal
handler is currently executing on that stack. If the thread is not currently
executing on the signal stack, the system arranges a switch to the alternate
signal stack for the duration of the signal handler's execution.
.sp
.LP
The  \fBstack_t\fR structure includes the following members:
.sp
.in +2
.nf
int   *ss_sp
long  ss_size
int   ss_flags
.fi
.in -2

.sp
.LP
If \fIss\fR is not \fINULL\fR, it points to a structure specifying the
alternate signal stack that will take effect upon successful return from
\fBsigaltstack()\fR. The \fBss_sp\fR and \fBss_size\fR members specify the new
base and size of the stack, which is automatically adjusted for direction of
growth and alignment.  The \fBss_flags\fR member specifies the new stack state
and may be set to the following:
.sp
.ne 2
.na
\fB\fBSS_DISABLE\fR\fR
.ad
.RS 14n
The stack is to be disabled and \fBss_sp\fR and \fBss_size\fR are ignored. If
\fBSS_DISABLE\fR is not set, the stack will be enabled.
.RE

.sp
.LP
If \fIoss\fR is not \fINULL\fR, it points to a structure specifying the
alternate signal stack that was in effect prior to the call to
\fBsigaltstack()\fR. The \fBss_sp\fR and \fBss_size\fR members specify the base
and size of that stack.  The \fBss_flags\fR member specifies the stack's state,
and may contain the following values:
.sp
.ne 2
.na
\fB\fBSS_ONSTACK\fR\fR
.ad
.RS 14n
The thread is currently executing on the alternate signal stack. Attempts to
modify the alternate signal stack while the thread is executing on it will
fail.
.RE

.sp
.ne 2
.na
\fB\fBSS_DISABLE\fR\fR
.ad
.RS 14n
The alternate signal stack is currently disabled.
.RE

.SH RETURN VALUES
.sp
.LP
Upon successful completion, \fB0\fR is return. Otherwise, \fB\(mi1\fR is
returned and \fBerrno\fR is set to indicate the error.
.SH ERRORS
.sp
.LP
The \fBsigaltstack()\fR function will fail if:
.sp
.ne 2
.na
\fB\fBEFAULT\fR\fR
.ad
.RS 10n
The \fIss\fR or \fIoss\fR argument points to an illegal address.
.RE

.sp
.ne 2
.na
\fB\fBEINVAL\fR\fR
.ad
.RS 10n
The \fIss\fR argument is not a null pointer, and the \fBss_flags\fR member
pointed to by \fIss\fR contains flags other than \fBSS_DISABLE\fR.
.RE

.sp
.ne 2
.na
\fB\fBENOMEM\fR\fR
.ad
.RS 10n
The size of the alternate stack area is less than \fBMINSIGSTKSZ\fR.
.RE

.sp
.ne 2
.na
\fB\fBEPERM\fR\fR
.ad
.RS 10n
An attempt was made to modify an active stack.
.RE

.SH ATTRIBUTES
.sp
.LP
See \fBattributes\fR(7) for descriptions of the following attributes:
.sp

.sp
.TS
box;
c | c
l | l .
ATTRIBUTE TYPE	ATTRIBUTE VALUE
_
Interface Stability	Standard
_
MT-Level	Async-Signal-Safe
.TE

.SH SEE ALSO
.sp
.LP
.BR getcontext (2),
.BR mmap (2),
.BR sigaction (2),
.BR ucontext.h (3HEAD),
.BR attributes (7),
.BR standards (7)
.SH NOTES
.sp
.LP
The value \fBSIGSTKSZ\fR is defined to be the number of bytes that would be
used to cover the usual case when allocating an alternate  stack area.  The
value \fBMINSIGSTKSZ\fR is defined to be the minimum stack size for a signal
handler.  In computing an alternate stack size, a program  should add that
amount to its stack requirements to allow for the operating system overhead.
.sp
.LP
The following code fragment is typically used to allocate an alternate stack
with an adjacent red zone (an unmapped page) to guard against stack overflow,
as with default stacks:
.sp
.in +2
.nf
#include <signal.h>
#include <sys/mman.h>

stack_t sigstk;
sigstk.ss_sp = mmap(NULL, SIGSTKSZ, PROT_READ | PROT_WRITE,
        MAP_PRIVATE | MAP_ANON, -1, 0);
if (sigstk.ss_sp == MAP_FAILED)
        /* error return */;
sigstk.ss_size = SIGSTKSZ;
sigstk.ss_flags = 0;
if (sigaltstack(&sigstk, NULL) < 0)
        perror("sigaltstack");
.fi
.in -2

